Specific steroid hormones act during fetal life to masculinize the infant, juvenile and adult behavior of genetic female mammals including nonhuman primates. The contribution these same hormonal substances make to the development of reproductive and social competence in genetic males has never been experimentally assessed in long gestational mammals. The hypothesis is being investigated that the phenotypic expression of masculine behavioral traits in both sexes can be quantitatively determined by the amount, kind, and duration of steroid hormone action on developing neural tissues. Experiments are proposed to determine the manner in which gonadal hormones act during fetal life to influence patterns of sexual and sex-related behaviors and to determine the manner in which these same chemical substances act to regulate the exression of these behaviors at later developmental ages. Biochemical methods have been developed for competitive protein binding and radioimmunoassay of a variety of gonadal hormones which permit quantitative measurement of steroids in peripheral venous blood or in endocrine gland venous effluent. In addition, methods are in use for the study of in vitro and in vivo metabolism and biotransformation of radio-labelled steroids in neural target tissues presumed to be involved in the mediation of behavior, for determining the biosynthesic capacity of the gonads at different developmental ages, and for assessing the ages at which specific steroids bind to nuclear components of neural cells. Biochemical and behavioral studies are carried out concurrently on a variety of species. This attempt to develop information within the same laboratory on a broad comparative front is being made for what such studies will contribute to our understanding of related problems in man.